1. Field of the Invention
The present invention relates to an optical head for example, for an optical recording/reproducing apparatus.
2. Description of the Prior Art
An optical head radiates light beams onto an optical recording medium while being focused thereon, and detects light beams therefrom. For this reason, when such an optical head is used, the beams must be split in different directions when they propagate forward and backward, and a focusing error must be detected. An astigmatic method is known as a method for detecting such a focusing error.
FIG. 1 shows a first conventional optical head which 1 comprises a cubic prism 2 combining rectangular prisms as a beam splitter, and a cylindrical lens 4 as an optical element for producing an astigmatism.
A light beam comprised of rays 6 emitted from a light source 5 passes through the cubic prism 2, a collimator lens 7, and an objective lens 8, and is incident on an optical recording medium 11. The rays 6 reflected from the optical recording medium 11 then pass through the objective lens 8 and the collimator lens 7, and are reflected by the prism 2 to be incident on a photodetector 12 through the cylindrical lens 4.
FIG. 2 shows a second conventional optical head 13, which has been disclosed in Japanese Patent Laying-Open No. 59-167863. In optical head 13, a plate-like half mirror 14 is used in place of the cubic prism 2 and the cylindrical lens 4.
The half mirror 14 is obtained by forming a translucent film on one surface of a transparent parallel-surface plate. When the half mirror 14 is arranged to be inclined with respect to converged light rays 6, as shown in FIG. 2, the rays 6 passing through the half mirror 14 cause an astigmatism. In other words, the half mirror 14 functions both as a beam splitter and to produce the astigmatism for detecting a focusing error.
However, in the first conventional optical head of FIG. 1, many optical elements (i.e., two triangular prisms constituting the cubic prism 2, and the cylindrical lens 4) are necessary for splitting the beams 6 and detecting the focusing error. Therefore, a relatively large number of individual parts and their assembly require high precision, resulting in increased cost of the optical head 1.
Although the second conventional optical head 13 of FIG. 2 requires only a small number of optical elements, so that its cost can be reduced, the half mirror 14 produces an undesirable coma in addition to the astigmatism.
FIGS. 3A to 3C show shapes of the beams comprised of rays 6 incident on the photodetector 12 when lenses having NA (numerical aperture values), of 0.47 and 0.14 are used for the objective lens 8 and the collimator lens 7, respectively, a 3-mm thick optical glass SF11 is used for the parallel-surface plate for the half mirror 14, and inclination of the half mirror 14 with respect to the optical axis of the beam 6 is set at 45.degree..
FIGS. 3A, 3B and 3C respectively show cases wherein the optical recording medium 11 is displaced toward the optical head 13 from an in-focus position by 10 .mu.m, wherein the recording medium 11 is located at the in-focus position and wherein the recording medium is separated from the in-focus position in the direction away from the optical head 13 by 10 .mu.m. As can be seen from FIGS. 3A, 3B, and 3C, the spot of the beam 6 on photodetector 12 is not symmetrical about the center of photodetecting portions a to d constituting the photodetector 12 due to the coma generated in the beam 6 incident on the photodetector 12 by inclined half mirror.
When the coma is generated in this manner, various problems are created for the optical head 13. For example, when the spot of the beam 6 is moved on the photodetector 12 for various reasons, the allowable range for deviation of the beam spot from a centered position is narrowed.
In addition, when only the objective lens 8 is moved for tracking servo control, the narrowed allowable range for deviation of the spot of the beam 6, correspondingly limits the movements of the objective lens 8, for maintaining correct tracking.
When a tracking error signal is obtained from a single spot of the beam 6, dependency of the signal on the recording medium 11 is exhibited to a considerable extent in addition to the above two problems, and the resultant signal is destabilized.
These problems narrow the margin for design of the optical head 13 associated with respective optical elements (e.g., not only of the half mirror 14, but also a nonsensitive band of the photodetector 12), and also in respect to the tracking error signal detection and the head actuator and so on. In addition, reliability of the optical head 13 is also degraded.